The present invention relates to power delivery circuit for supplying load current to a load, and more particularly to such a circuit including means to detect a condition of the current, such as an over-current condition.
Power delivery circuits first for supplying load current to a load and including over-current detection means are known. See for example, U.S. Pat. Nos. 4,705,997 and 4,654,568. Such circuits typically include a current switch between a load and ground so that the switch is placed on the low potential side of the load. An over-current detection means is then responsive to the current level in the low potential side switch. Such circuits typically require a dual-polarity voltage supply to provide a negative bias to an over-current detection device typically incorporating operational amplifier circuitry.
In environments where only single-polarity voltage supplies are provided, such as in automobiles, the above-described prior art circuitry requiring dual-polarity voltage supply cannot be used. It is also known to use current-sensing MOSFETs in the high side of a power switch. However, these devices employ very complex control circuitry.
It is, accordingly, an object of the invention to provide a power delivery circuit having a switch located on a high potential side of the load and including simple and reliable circuit means to sense a current condition, such as an over-current condition in the load. The power delivery circuit may include means to turn off the current through the switch upon detection of an over-current condition to prevent destruction of the switch or of the load. The foregoing type of circuit is particularly suitable for use with a single-polarity voltage supply, such as typically provided in automobiles. Another object of the invention is the provision of simple and reliable circuit means to sense the condition of current through a current switch to a load and which functions accurately within a relatively narrow tolerance range and over a relatively broad operating range.
A further object of the invention is to provide a power delivery circuit with over-current protection that functions accurately and reliably despite large changes in ambient temperature, such as is typically encountered in an automobile.
Another object of the invention is to provide a power delivery circuit with over-current protection that can be inexpensively manufactured with the use of readily-available discrete components.
In accordance with a preferred form of the invention, the power delivery circuit for supplying load current to a load is provided. The circuit includes a power node for connection to a source of electrical power, a load node for connection to a load, and a high potential current path for supplying electrical current from the power node to the load node. The high potential current path includes a current switch whose switching state is controlled by a control signal on an associated control terminal. The power delivery circuit further includes current sensing circuitry for reliably sensing the level of load current over a wide operating range. The power delivery circuit may further include a single, highly reliable over-current detection circuit responsive to the sensed condition of current in the high potential current path. The over-current detection circuit is for detecting when the current in the high potential current path exceeds a predetermined level. The power delivery circuit, preferably, further includes current turn-off means for disabling current flow to the load when an over-current condition is detected.
In a particularly preferred embodiment of the invention for providing accurate and reliable over-current detection, the current switch in the high potential current path comprises a multicellular device with a plurality of terminals including a first main current terminal connected to the power means, a second main current terminal connecting a majority of device cells to the electrical load, a first auxiliary terminal connected at one end to a minority of device cells so as to provide a current generally proportional to the main device current, and a second auxiliary terminal connected at one end to the majority of device cells. The over-current detection circuit in this embodiment preferably comprises one branch connected between the first auxiliary terminal and a reference potential such as ground and including, in serial circuit, a bias-controlled transimpedance device such as a transistor and a resistor placed between the transimpedance device and the ground and providing a voltage at one end approximately proportional to the main device current, and another branch connected between the second auxiliary terminal and the ground and providing a control signal for the transimpedance device in the first branch.
The transimpedance device in the foregoing over-current detection circuit beneficially couples the current from the high-potential-side first auxiliary node to the resistor that is at low potential. This permits such resistor to be selected so as to provide a desired sense voltage indicative of a particular load current. Moreover, the branch connected between the second auxiliary terminal and the ground for providing the control signal preferably includes a transistor having its base cross-connected to the transimpedance device in the first branch and further having its base shorted to its collector.